Heat
Definition
Energy moving from a hotter object to a colder one because of the temperature difference.
Symbol: · Unit: joule (J)
Technical definition
Energy in transit driven by a temperature gradient, denoted Q. Heat is not a property an object 'contains' — objects contain internal (thermal) energy; heat is the transfer process.
Example
Hold a hot mug and your hands warm up — thermal energy flows as heat from the mug (hotter) to your skin (colder) until the temperatures even out. Insulate the mug and you slow that flow.
Heat is not temperature, and objects do not 'contain heat.' A lukewarm swimming pool holds far more thermal energy than a red-hot nail, because energy depends on mass too. In physics, heat is the flow itself — energy in transit — not the stuff flowing. Saying 'the heat in this room' really means its thermal energy.